What is a foetal specialist scan? And do you need one?

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Foetal medicine specialists are usually gynaecologists who have studied further to become sub-specialists. They offer routine ultrasound screening, follow up screening (for confirmation if an abnormality is suspected) and perform invasive procedures like amnios and chorionic villus sampling. Their job is to identify and test for abnormalities.

While not everyone has to see a foetal medicine specialist, it is recommended that all pregnant women should be counselled about screening and told what tests are available. The problem is that abnormalities can, and do, occur in low risk women and it is these that need to be identified. Some general gynaecologists have registered with the foetal medicine foundation and are doing their own first trimester scans by measuring the nuchal translucency (NT) and doing the blood tests. Others offer all patients a referral to a specialist unit. This depends on what is available in your area, and on you and your doctor’s preference.

If only select patients are referred in, criteria would include maternal age, history of previous abnormalities, family history, exposure to certain medications in pregnancy and rhesus negative moms. For routine screening two assessments are recommended – the first trimester screen and the second trimester scan.

First trimester screen

  • WHEN: 11 to 13 weeks and four days, or when the foetus measures 45-84mm.
  • AIM: This screen aims to detect an increased risk for major abnormalities in the foetus, specifically chromosomal abnormalities such as Down Syndrome. Certain other non-chromosomal problems can also be detected.
  • WHY: As these abnormalities are often severe and cannot be corrected, many couples may opt to end the pregnancy. This is safest if done earlier in pregnancy. Many parents opt not to terminate even if an abnormality is confirmed, but can then plan for the birth and prepare themselves and family as to what to expect.
  • HOW: A baseline risk for various chromosomal abnormalities based on maternal age is modified by biochemical factors (determined by blood tests) and sonar findings. Two hormone levels in the blood are measured, PAPP-A and hCG. These are compared to a reference range specific to gestational age. On sonar, some of the things that are examined are the nuchal translucency or NT, the nasal bone and the twelfth rib. It is important to remember that this is a screening test – if the results from this test put you in a high risk group then you will be offered an amniocentesis to confirm the screening test result.

Second trimester scan

  • WHEN: 18 to 23 weeks.
  • AIM: To identify structural abnormalities, or things that haven’t developed properly.
  • WHY: Often these abnormalities are isolated and can be repaired. However,it is important to see if there are a cluster of abnormalities as seen in certain syndromes. If the abnormality is identified it helps to plan for the care of the baby after delivery, for example, babies with a heart problem can be delivered where there is a specialised paediatric cardiac unit.
  • HOW: This screening is done using ultrasound only. A level three sonographer will maximise the amount that is picked up.
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